Method For Speed Based Control Of An Implement Steering System

ABSTRACT

A method for disabling the implement steering system of a towed implement includes monitoring a speed of the implement and if the speed exceeds a predefined limit, automatically disabling the implement steering system.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention is generally directed to steering of a farmimplement and, more particularly, to a method for controlling animplement steering system.

Many tillage, seeding, and planting equipment are designed to be drawnbehind tractors and the like. Most of these towed devices are steeredprimarily by the tractor and do not have their own steering mechanisms.

Increasingly however, farm implements and other towed devices are beingequipped with steering systems that allow the towed device to steerindependent of the towing device, e.g., tractor. Seeding implements, forexample, equipped with implement steering systems, allow the seedingimplement to remain aligned with the tractor at all times during seedingin order to obtain straight and even seed rows. However, when thetractor is traversing in a direction perpendicular to the slope of theland, the implement steering system allows the seeding implement to moveindependently of the tractor to avoid the implement from “side slipping”or moving in the direction of the slope thereby falling out of alignmentwith the tractor. In this regard, the implement steering system providesfor consistent row spacing.

Another circumstance when it is desirable to have an independentlysteerable implement is when the tractor must maneuver the implementaround an obstruction such as a slough, telephone pole, large boulderand the like. If the towed implement does not have independent steering,the tow vehicle must make a wider turn which will result in a tendencyfor the towed implement to “cut corners” thereby, in the case of aseeding implement, the seed openers are more apt to twist instead oftravel straight ahead, which is the intended use.

Thus, a towed implement having independent steering would be able tosteer itself back into alignment with the tractor and minimize the towedvehicle's arc of travel. This can be accomplished by equipping the towedimplements with at least one steerable surface engaging wheel, and morepreferably, with two steerable surface engaging wheels integrated aspart of the main frame of the towed implement. Steering can either beautomatically controlled by means of a steering angle sensor, globalpositioning sensor (GPS) or could be operator controlled.

One of the drawbacks of conventional implement steering systems is thatthe system, once activated, remains activated until it is manuallydeactivated. This can be problematic if the implement steering system isengaged when conditions are not well-suited for auto-steering. Forexample, most implement steering systems are designed for use at lowerspeeds, such as the speeds during field operation. When the implement isbeing transported, which is typically done at higher travel speeds,conventional implement steering systems do not have the response timethat is necessary to make timely corrections to the implement'sposition, thus are typically disabled.

The present invention provides an implement steering system thatself-disables if a high travel speed is detected. Once the implement hasreturned to a safe auto-steering speed, the implement steering system isautomatically enabled. In this regard, the invention does not requiremanual shut-off of the implement steering system at higher travelspeeds. Similarly, the invention does not require an operator tomanually enable the implement steering system when safe operating speedsare obtained.

The present invention also provides for auto-centering of the steeringsystem when in transport. As implements become larger, navigation aroundturns becomes increasingly difficult. Having an independent steeringsystem provides needed assistance for negotiating turns. This alsorequires drives attention when negotiating back out of the turn. In somecases this leads to driver distraction. The present invention providesan auto-center feature that allows the steering system to self-alignwhen a target ground speed is detected in the event the operator becomesdistracted with forward operation.

The implement steering system of the invention is designed to remainrelatively quiet when the implement is being towed during in-field use,i.e., during active seeding. Although relatively quiet as the implementis being towed along rows, the implement steering system of the presentinvention is designed to independently auto-center the steering systemof the implement at headland turns to correct for field impacts,internal leaks, thermal changes and other conditions that may havecaused misalignment of the implement. In this regard, the implementsteering system has a self-centering feature that is active duringheadland turns and at other turning instances.

In addition to accounting for unsafe auto-steering conditions, thepresent invention also provides an implement steering system that isselectively enabled and disabled based on the mode of operation of theimplement.

It is an object of the invention to provide an implement steering systemthat is disabled automatically at higher transport speeds.

It is also object of the invention to provide an implement steeringsystem that automatically centers itself relative to the definedcalibration value at headland turns of the towing vehicle.

It is another object of the invention to provide an implement steeringsystem that automatically disables itself if a steering angle of theimplement is outside an acceptable range of values.

It is yet a further object of the invention to provide audio and/orvisual indications to an operator conveying the status of the implementsteering system for the implement.

Other objects, features, aspects, and advantages of the invention willbecome apparent to those skilled in the art from the following detaileddescription and accompanying drawings. It should be understood, however,that the detailed description and specific examples, while indicatingpreferred embodiments of the present invention, are given by way ofillustration and not of limitation. Many changes and modifications maybe made within the scope of the present invention without departing fromthe spirit thereof, and the invention includes all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS (ADD UPDATED FIGURES)

Preferred exemplary embodiments of the invention are illustrated in theaccompanying drawings in which like reference numerals represent likeparts throughout.

In the drawings:

FIG. 1 is an isometric view of a carrier frame of an agriculturalimplement having an implement steering system according to one aspect ofthe invention;

FIG. 2 is an enlarged top isometric view of a portion of the carrierframe shown in FIG. 1;

FIG. 3 is a schematic diagram of a control system for an implementsteering system according to one aspect of the invention;

FIG. 4 is a schematic diagram of a hydraulic circuit for the implementsteering system shown in FIG. 3;

FIG. 5 is a flow chart setting for the steps of a process forauto-centering an implement as it is being towed in transport by atowing vehicle;

FIGS. 6A and 6B are flow charts setting for the steps of a process fordisabling auto-centering of an implement and allowing manual centeringof the implement based on a travel speed of the towing vehicle; and

FIG. 7 is a flow chart setting for the steps of a process forautomatically centering a towed implement during in-field use of thetowed implement.

DETAILED DESCRIPTION

The present invention with be described with respect to an implementsteering system and, more particularly, to a set of control processesfor software based control of the implement steering system of a farmimplement, such as front folding planter. An exemplary front foldingplanter is described in U.S. Pat. No. 7,469,648, the disclosure of whichis incorporated herein by reference. The present invention is alsoapplicable for software control of the implement steering systems ofother towed devices, such as a hay wagon, seeders, tillage implements,trailers, transport trailers, work trailers, flat beds, freighttrailers, and the like. Moreover, while the invention will be describedwith respect to a method of controlling an implement steering system, itwill be appreciated that the method can be embodied in a computerprogram, software, or other computer executable code that is executed bya processor, controller, or other electronic control unit, such as asteering electronic control unit.

Additionally, while the present invention will be described with respectto an implement steering system for a front folding planter in which aseries of hydraulic cylinders are used to control folding/unfolding andsteering of the planter, it is understood that the invention may also beused to control various types of implement steering systems.Additionally, the invention may be used with towed device equipped withguidance systems, such as those that use GPS or similar navigationsystems to auto-control movements of the towed device, such as describedin U.S. Pub. No. 2005/0015189 and U.S. Pat. No. 7,147,241, thedisclosures of which are incorporated herein.

Turning now to FIG. 1, a representative front folding planter has acarrier frame 10 that is centrally positioned between a pair of wingbooms (not shown) and is used to support a central bulk fill hopper (notshown) as known in the art. The carrier frame 10 includes a pair offorward mounting arms 12, 14 for mounting the carrier frame 10 to acenter frame tongue (not shown) or other structure for hitching thefront folding planter to a tractor or other towing vehicle. The carrierframe 10 is supported by two pairs of wheels 16, 18 that are mounted tothe carrier frame 10 in a conventional manner using wheel mounts 20, 22,respectively. The mounting arms 12, 14 also couple to the wheel mounts20, 22, respectively.

The pairs of wheels 20, 22 are steerable by an implement steering system50, as will be described more fully below. To facilitate this steering,steering cylinders 24, 26 include barrels 28, 30 that are connected tothe carrier frame and rams 32, 34, respectively, that are connected tothe wheel mounts 20, 22, respectively. The wheel mounts 20, 22 arepivotally coupled to the rams 32, 34 such that as the rams are extendedand retracted, the respective wheels pairs 16, 18 turn about verticalaxes 36, 38.

With additional reference now to FIG. 2, a carrier position sensordevice 40 that provides feedback regarding the position of the carrierframe relative to a longitudinal traveling axis of the implement. Thecarrier position sensor device 40 includes a sensor 42 and apotentiometer 44 that is interconnected to the sensor 40 and the wheelmount 20. The sensor 42 is mounted to a fixed portion of the carrierframe, i.e., crossbar 46, and the potentiometer 44 is connected to thewheel mount 20. Thus, when the wheel mount 20 pivots, the angle of thepotentiometer 44 relative to the sensor 40 will change, which allows thesensor 40 to measure the angle of the wheel mount 20 relative to thecarrier frame 10.

Turning now to FIG. 3, a general layout of a control system for animplement steering system according to one embodiment of the inventionis shown. In general, the control system 50 includes a processing unitor “steering electronic control unit (ECU)” 52 that receives informationfrom various sensors, such as position sensors 54, 56, e.g., carrierframe sensor device 40, pressure sensors 58, 60, and a ground speedsensor 62. It is also contemplated that some sensor input values may bereceived by the steering electronic control unit (ECU) via CAN buscommunication with other on-board ECUs. To effectuate steering of theimplement, the implement is equipped with a series of hydrauliccylinders. For purposes of description, the implement will be consideredto have two steering cylinders—left and right steering cylinders. Itshould be noted that the implement may also have additional hydrauliccylinders such as those used to raise and lower the implement and foldand unfold the implement.

The pressure sensors 58, 60 measure the pressure in the supply andreturn hydraulic fluid lines that couple the hydraulic system of theimplement to the hydraulic system of the towing vehicle. In this regard,in one embodiment, the hydraulics of the implement are run off thehydraulic system of the towing vehicle; however, it is contemplated thatthe implement could have an independent hydraulic system, i.e.,hydraulic fluid source and pump(s). The ground speed sensor 62 may bemounted to measure the travel speed of the tractor or may be mounted tomeasure the travel speed of the implement. In one embodiment, the groundspeed sensor 62 is the same sensor that is used to collect travel speedinformation that is displayed on a speedometer of the tractor. In analternate embodiment, the ground speed sensor 62 is a separate sensorthat measures the travel speed of the tractor. Alternatively, the groundspeed sensor may also be an output from a GPS receiver. The positionsensors 54, 56 are a steering axle sensor and a carrier position sensor,respectively. In this regard, position sensor 54 is mounted to thesteerable axle of the implement and position sensor 56 measures theangle between the carrier 10, FIG. 1, and a center frame member (notshown) of the implement. Additionally, it is understood that other typesof position sensors, such as GPS sensors, could be used to determine therelative positions of the steerable axle and the center frame.

As will be described in greater detail with respect to FIG. 4, theimplement steering system 50 includes a hydraulic circuit 64. System 50also includes a display unit 66 contained within the operator cab (notshown) of the implement on which messages regarding the status of theimplement steering system can be displayed to an operator. Also, thedisplay unit can incorporate an alarm to signal the operating status.Alternatively, an alarm 68 is also provided that can be sounded by theECU 52 to signal the operating status of the implement steering system10. The display unit 66 may incorporate a status indicator that is usedto notify the operator of the current operating state of the implementsteering system. Alternatively, the implement steering system 50 alsoincludes a status indicator 70 that is caused to flash by the ECU whenthe implement steering system is active.

Steering of the implement and, more particularly, a steerable axle ofthe implement is controlled by a hydraulic control circuit 64, which isschematically illustrated in FIG. 4. The hydraulic control circuit 64has two solenoid valves 72, 74, and two proportional valves 76, and 78.Valves 72 and 74 are associated with supply and return lines 80 and 82,respectively. In this regard, when valve 72 is open, hydraulic fluid mayflow in the supply line and when closed hydraulic fluid flow, includingback flow, is prevented. In a similar manner, valve 74 when open allowshydraulic fluid to flow in the return line and when closed prevents theflow of hydraulic fluid in the return line. It will also be appreciatedthat the valves 72, 74 when closed prevent back flow of hydraulic fluidand therefore can be used to maintain pressure in the hydraulic circuitand also serves to isolate the steering circuit from the core implementhydraulics.

Valves 76, 78 are variable and control the flow of hydraulic fluidthrough cylinders 84, 86, respectively. Cylinders 84, 86, as will bedescribed, are operable to effectuate left and right turns of theimplements steerable axle. The solenoid valves are capable of receivinga variable electrical signal from the ECU 52. Each solenoid valve has anelectromagnet (not shown) such that when an electrical signal isreceived, the electromagnet position that shifts the position of thevalve proportionally to alter the flow of hydraulic fluid. In thisregard, the valves 76, 78 provide signals, which in this case, take theform of a slug of hydraulic fluid, to their respective hydrauliccylinders to cause retraction or extension of the cylinders. Asdescribed with respect to FIGS. 1 and 2, each cylinder has a barrel anda rod (that are connected to the frame of the implement, and moreparticularly interconnected between a stationary component of the frameand a movable component of the frame, such as between the fixed frame ofthe implement and a steerable axle. Thus, when the hydraulic pressure inthe barrel changes, the rod will extend or retract thereby causing acorresponding movement in the movable component to which the cylinder isconnected. It will therefore be appreciated that the ECU 52 can, inresponse to feedback from the aforementioned sensors, control the flowof hydraulic fluid to the cylinders 84, 86 and thus electronically steerthe implement. In one preferred embodiment, the hydraulic circuit 64 hascheck valves, CV1 and CV2, inline between the solenoid valves 76, 78 andcylinders 84, 86 to serve as a lock when implement steering is notactivated.

The ECU 52 is programmed to control the steering of the implement basedon feedback received from the position, speed, and pressure sensors. Inthis regard, the invention provides a software-based control of theimplement steering system. As will be described with respect to FIGS. 5,6 and 7, the ECU 52 and display unit 66 executes various sets ofexecutable code to operate the implement steering system according toone of three processes. One of these processes, which is illustrated inflow chart form in FIG. 5, is designed to automatically center theimplement during transport. It will be appreciated that during transportthe implement is in the folded position.

To initiate automatic steering correction of the implement duringtransport, process 88 causes the ECU to first determine the vehiclespeed. If the vehicle speed has exceeded the prescribed limits and theimplemented steering is not in the centered position, the automaticcorrection process initiates. Once initiated, the ECU 52 determines thestates of the two pressure sensors 58, 60 at blocks 90 and 92. Pressuresensor 58 provides feedback to the ECU regarding the presence ofpressure in the supply line at block 90 and pressure sensor 60 providesfeedback to the ECU regarding the presence of pressure in the returnline at block 92. If there is a non-trivial pressure in either thesupply line or the return line, the ECU 12 activates solenoids 72 and 74to allow hydraulic fluid flow to and from the hydraulic circuit 64 andsimilarly activates either steering solenoids 76 or 78 to enablesteering of the implement at blocks 94 and 96. To make the operatoraware that the implement steering system has been enabled, the ECU 12and display unit flash a status icon (not shown) on the display screen.

On the other hand, if no pressure is measured in either the supply orreturn lines, the ECU 52 causes the display unit 66 to display a messageindicating that the implement steering system has not been enabled atblock 98. For example, the display unit 66 displays the message“IMPLEMENT STEERING AXLE NOT IN CENTER POSITION. STOP VEHICLEIMMEDIATELY AND CENTER.” This message conveys to the operator that theautomatic steering system is disabled and not capable of steering theimplement. Accordingly, the operator must move the hydraulic remotecontrol lever to an active position to pressurize the hydraulic system.Once pressure is detected at either pressure sensor 58 or pressuresensor 60, the ECU 52 will provide control signals to solenoids 72, 74,and 76 or 78.

Pressure sensor 58 senses pressure in the supply line and pressuresensor 60 senses pressure in the return line. In this regard, ifpressure sensor 58 detects a non-trivial pressure in the supply line,the ECU 52 will activate solenoids 72 and 74 and further activatesolenoid 78 to enable left turn corrections and active solenoid 76 toenable right turn corrections. On the other hand, if the pressure sensor58 does not detect pressure in the supply line, but pressure sensor 60detects pressure in the return line, then solenoids 72 and 74 areactivated and solenoid 78 is activated to enable right turn correctionsand solenoid 76 is activated to enable left turn corrections. It willthus be appreciated that whether the fluid pressure in the supply andreturn lines has a supply bias or a return bias will determine whethersolenoids 76, 78 are used to enable left or right turn corrections.

With solenoids 76, 78, 72 and 74 activated or open, the ECU 52 thenreceives feedback from steering axle sensor 54 at block 100. If the ECU52, from the feedback provided by the steering axle sensor 54,determines that a steering axle change has taken place within aprescribed time, e.g., 250 ms, after the solenoids have been activated,the ECU 52 then determines if the change in the steering axle was in thecorrection direction at block 102. That is, if a right turn correctionwas needed, was a right turn correction made, for example. It isappreciated that the correctional direction is determined by ECU 52using the current sensor position relative to the target center positionas determined via a prior manual center calibration. If the steeringaxle change was not in the correction direction, the ECU 52 reverses theactivation of solenoids 76, 78 at block 104. In either instance, the ECUthen determines from feedback from the steering axle sensor 54 if thesteering axle angle is within a predefined range, e.g., >−0.3° and<+0.3°, at block 106. If not, the process loops back to block 90. If so,however, the ECU 52 disables the implement steering system bydeactivating the steering solenoids 72, 74, 76 and 78 at block 108. Itis also appreciated that the ECU 52 and the display unit 66 providenotification to the operator that the implement steering system has beendisabled.

The ECU 52 them compares feedback from the speed sensor 62 to a maximumspeed limit for implement steering at block 110. If the actual groundspeed of the implement, as measured by the ground speed sensor 62, orequivalent speed source, is less than the maximum speed limit, e.g.,14.5 kph, for a predefined time period, e.g., two seconds, the ECU 52enables manual operation of the implement steering system in accordancewith a manual control process, FIGS. 6A and 6B, steering system at block112. If the speed does not remain below the maximum speed limit for thepredefined limit, the process loops back to block 108. In this regard,if the implement is traveling above a safe implement steering speedlimit, steering of the implement, either automatically or manually, willbe prevented so no undesirable steering axle angle can occur.

Referring now to FIGS. 6A and 6B, the manual control process 114 beginswith receiving feedback from the pressure sensors to determine if thereis pressure in either the supply line or the return line at block 116.If so, the steering solenoids remain disabled and a suitable message isdisplayed on the display unit at block 118. One exemplary message may be“CYCLE HYDRAULIC REMOTE LEVER TO FLOAT AND THEN PLACE IN NEUTRAL BEFOREACTIVATING IMPLEMENT STEERING”. If, on the other hand, the pressuresensors provide feedback indicating no pressure in the supply or returnlines, the ECU 52 determines, from feedback provided by the ground speedsensor 62, or equivalent speed source, if the travel speed of theimplement is less than the maximum speed limit, e.g., 14.5 kph, forsteering the implement at block 120. If the maximum speed limit is beingexceeded, the ECU 52 maintains the steering solenoids in the disabledstate and causes the display unit to display a message that the speed ofthe implement is too fast for implement steering at block 122. If thespeed is below the speed limit, the ECU 52, from feedback received fromthe carrier position sensor 56, determines if the carrier is at leasteighty percent of a maximum carrier position at block 124. If not, theECU 52 maintains the steering solenoids in their disabled state andcauses the display unit to display a message that the carrier positionis too low for implement steering at block 126. On the other hand, ifthe implement is sufficiently raised, the ECU enables manual control ofthe steering system at block 128, which includes disabling alllift/fold/marker solenoids (not shown) and enabling the steeringsolenoids 72, 74, and 78 at block 130. It should be noted that whenmanual control of the steering system is activated, solenoid 76 remainsdisabled.

With manual control of the implement steering system activated, the ECU52 continues to receive feedback from the ground and position sensors.In this regard, the ECU 52 determines if the carrier position sensor iswithin a valid range at block 132. If so, the ECU 52 then determines ifthe steering angle sensor is within a valid range at block 134. If thecarrier position sensor is outside the valid range, the process proceedsto block 136 whereupon the ECU 52 disables the steering solenoidseffectively disabling implement steering and causes the display unit todisplay a corresponding message. Similarly, if the steering angle sensoris outside the valid range, the ECU 52 disables the steering solenoidsand causes the display unit to display a corresponding message regardingthe carrier position sensor being out of range at block 138.

If the steering angle sensor is operating properly, the process proceedsto block 140 whereupon the ECU 52 determines if the ground speed of theimplement is greater than the ground speed limit for implement steering.If the ground speed is below the speed limit, the steering solenoidsremain enabled at block 142 and the ECU 52 proceeds to block 144 todetermine if the display has been powered down. If the speed limit hasbeen exceeded for a predefined time period, e.g., two continuousseconds, the ECU 52 then determines if the angle of the steerable axleis outside a predefined range, e.g., >−0.3° and <0.3°, at block 146. Ifit is, the ECU 52 proceeds to execute the steps of an auto-centeringprocess at block 148. If the angle is inside the range, however,implement steering is disabled at block 150. In this regard, manualsteering of the implement is not allowed when the implement is travelingat higher speeds and the angle of the steerable axle is inside theaforementioned range. The ECU 52 then proceeds to block 152 anddetermines if the travel speed of the implement is less than thesteering speed limit. If not, the process loops back to block 150. Ifthe travel speed of the implement has dropped below the maximum speedlimit, the ECU 52 returns to block 116 whereupon the manual controlactivation process begins and will automatically activate uponconditions beginning at block 116, thus eliminating the need foroperator intervention. It will be appreciated that the auto-centeringprocess can be carried out in a number of ways but generally includespulsing the steering solenoids 76, 78 to extend or retract the steeringcylinders until the misalignment if the implement with the towingvehicle is corrected.

The present invention also includes a method for controlling implementsteering during planting. This method, which may also be embodied incomputer executable code that can be executed by the ECU 52, is designedto provide automatic centering of the implement during planting andthereby provide drift correction as well as off center correction thatmay occur because of pressure leakage as the implement is towed alongthe planting surface. The process also provides automatic centering ofthe implement at headland turns.

Turning now to FIG. 7, the automatic centering during planting process154 begins at block 156 whereupon the ECU 52 determines if the framecontrol (not shown) is in the planting position. If the frame control isnot in the planting position, the process loops back to block 156.However, if the frame has been lowered into the planting position, theECU 52 proceeds to determine if the angle of the steerable axle iswithin a first predefined range; namely, ≦2.0° or ≧−2.0°, at block 158.If the angle is outside the range, the ECU 52 causes the display unit 66to display a fault message that the implement is too far off center anddisables the steering solenoids at block 160. Once the notification hasbeen acknowledged, the ECU 52 advances to block 162 whereupon the ECU 52takes no corrective action by ensuring that the steering solenoidsremain disabled. If the angle of the steerable axle is within the firstpredefined range, the ECU 52 then determines if the steerable axle isoutside a second predefined range, e.g., >−0.4° and <0.4° , at block164. If so, the ECU 52 moves to block 162. If not, the ECU 52 enablesthe frame lift solenoids 166 and enables the steering solenoids so thatthe implement may be automatically centered at block 166. Specifically,solenoids 72, 74, and 78 are enabled for left turn correction andsolenoids 72, 74, and 76 are enabled for right turn correction. With thesolenoids activated to enable automatic centering of the implement, thedisplay unit 66 status indicator is activated at block 166. In onepreferred embodiment, the ECU 52 provides control signals to thesteering solenoids such that a one-half degree change in steering axleposition is made within a defined time period, i.e., 1000 milliseconds,until the angle of the axle is within the first predefined range. Inthis regard, after the steering system has been activated, the processreturns to block 156.

It will therefore be appreciated that the present invention provides aprocess for controlling the steering of a steerable implement. Theprocess, which can be embodied in computer executable code for executionby an electronic control unit, includes various sub-processes orroutines that disable the implement steering system if the angle of thesteerable axle is outside a predefined range or if the travel speed ofthe implement exceeds a predefined limit. The process allows manualcontrol of the implement steering system under certain conditions butwill also disable manual control is the implement is too far out ofposition or traveling at a speed in excess of a control limit.

Many changes and modifications could be made to the invention withoutdeparting from the spirit thereof. The scope of these changes willbecome apparent from the appended claims.

1. A method for enabling manual control of an implement steering system,the method comprising: electronically monitoring a ground speed of thefarm implement as it is being towed by a prime mover with the implementsteering system of the farm implement disabled; and enabling manualcontrol of the implement steering system if the ground speed is below apredefined speed limit.
 2. The method of claim 1 further comprisingenabling manual control of the implement steering system if theimplement is in a raised position.
 3. The method of claim 2 furthercomprising issuing an electronic message that the implement is too lowfor manual control of the implement steering system if the implement isdetermined to not be in the raised position.
 4. The method of claim 2further comprising disabling a lift system and a fold system of theimplement when manual control of the implement steering system isenabled.
 5. The method of claim 2 further comprising comparing theground speed of the implement to the predefine speed limit after manualcontrol of the implement steering system has been enabled and disablingmanual control of the implement steering system if the ground speedexceeds the predefined speed limit for a predefined period of time. 6.The method of claim 5 further comprising determining a degree ofmisalignment of the implement after manual control of the implementsteering system has been disabled and if the degree of misalignment iswithin a predefined range, then automatically centering the implementrelative to the towing vehicle.
 7. The method of claim 6 furthercomprising disabling the implement steering system if the degree ofmisalignment is outside the predefined range.
 8. The method of claim 7further comprising re-enabling manual control of the implement steeringsystem only after the ground speed of the implement falls below thepredefined speed limit for a predefined time period.
 9. The method ofclaim 1 further comprising flashing a status indicator when manualcontrol of the implement steering system is enabled.
 10. An implementsteering system for use with a towable implement having a frame, thesystem comprising: a plurality of solenoid control valves configured tocontrol fluid flow to a plurality of cylinders adapted to steer asteerable axle of the towable implement; a carrier position sensor thatmeasures a height of the frame of the implement; and an electroniccontrol unit (ECU) configured to execute a set of instructions containedon computer readable medium that when executed causes the ECU to receivefeedback from the carrier position sensor, and wherein the ECU thenelectrically activates the plurality of solenoid control valves if theframe is in a raised position to enable steering of the steerable axle.11. The implement steering system of claim 10 further comprising aground speed sensor that measures a travel speed of the implement, andwherein the ECU is configured to activate the plurality of solenoidcontrol valves only if the ground speed of the implement is below acontrol speed limit.
 12. The implement steering system of claim 11wherein the ECU is further configured to disable manual steering of thesteerable axle if the ground speed of the implement exceeds the speedcontrol limit for a predefined period.
 13. The implement steering systemof claim 12 wherein the ECU is further configured to provideauto-centering command signals to the plurality of solenoid controlledvalves if the ground speed exceeds the speed control limit for thepredefined period and if misalignment between the implement and a towingvehicle is within a predefined misalignment range.
 14. The implementsteering system of claim 13 wherein the ECU is configured to disable thesolenoid controlled valves if the ground speed exceeds the speed controllimit for the predefined period and if the misalignment is outside thepredefined misalignment range to disable steering of the steerable axle.15. The implement steering system of claim 10 further comprising a lampand wherein the ECU is further caused to flash the lamp when thesolenoids are activated.
 16. A computer readable storage mediumcontaining a set of instructions in a computer readable format storedthereon that when executed by an electronic control unit (ECU) cause theECU to control operation of an implement steering system for animplement connected to a towing vehicle, by: electronically determininga ground travel speed of the implement as it is being towed by thetowing vehicle; and if the ground travel speed is below a speed limit,electronically monitoring a position of a carrier frame of the implementas it is being towed by the towing vehicle; determining if the carrierframe is in a raised position; if so providing control signals to aplurality of solenoid controlled valves to allow hydraulic fluid to flowto and from a plurality of hydraulic cylinders coupled to the carrierframe and a steerable axle of the implement to enable manual operationof the implement steering system.
 17. The computer readable storagemedium of claim 16 wherein the set of instructions further causes theECU to provide control signals to the plurality of solenoid controlledvalves to provide automatic centering of the implement relative to thetowing vehicle if the ground speed of the implement exceeds the speedlimit for a predefined time period and misalignment between theimplement and the towing vehicle is within a predefined margin ofcorrection.
 18. The computer readable storage medium of claim 17 whereinthe set of instructions further cause the ECU to disable the implementsteering system if the ground speed of the implement exceeds the speedlimit for the predefined time period and the misalignment of theimplement with the towing vehicle is outside the predefined margin ofcorrection.